Engines are known which include a direct injection valve configured to directly inject fuel into a cylinder, and a port injection valve configured to inject fuel into an intake port.
Such engines include a control device configured to cause fuel to be injected from only one of the direct injection valve and the port injection valve, or from both of these valves in accordance with the operation state of the engine.
Especially in engines including the direct injection valve, pre-ignition tends to occur which is a phenomenon in which fuel is spontaneously ignited in a cylinder before ignited by an ignition device such as an ignition plug. If pre-ignition occurs, pressure rapidly increases in the combustion chamber, so that the shock wave collides with the piston or the inner periphery of the cylinder. This collision further increases the temperature in the cylinder, so that the engines may become unable to function properly.
Especially in recent high compression ratio engines, the higher the compression ratio is, the more likely low-speed pre-ignition is to occur in the low-revolution, high-load operation range of the engines. In case of engines including a supercharger, the higher the supercharging pressure is, the more likely low-speed pre-ignition is to occur in the low-revolution, high-load operation range of the engines. Therefore, it is necessary to take a countermeasure to prevent this phenomenon.
As one of the causes of pre-ignition occurrence, it is said that deposits accumulated in the combustion chamber or liquid drops of lubricant oil dispersed from the inner peripheral wall of the cylinder ignite as the temperature of the combustion chamber rises so that end gas is self-ignited.
As a method for preventing pre-ignition, for example, there is a method for lowering the temperature of air-fuel mixture, or a method for lowering the oxygen concentration in air-fuel mixture. As a method for lowering the temperature of air-fuel mixture, for example, there is a method for lowering the supercharging pressure of air-fuel mixture by waste gate valve control, etc., or a method for lowering the actual compression ratio by retarding an intake valve by variable valve timing mechanism.
The below-identified Patent Documents 1 and 2 disclose that as a measure after the occurrence of pre-ignition in the high-revolution, high-load operation range of the engine, the enrichment of air-fuel ratio, the retardation of the closing timing of the intake valve, or the retardation of the injection timing of a portion of the fuel, etc. is performed in a stepwise manner.